Garry,
Have you searched on this yet? There have been many, many posts on this issue with a good deal of input from the professional techs. Try a search using a combination of terms, like "asr m119" or some such (since this is usually a problem wiht the M119 engine and mostly with ASR-equipped cars. (I THINK your 400E still has an electronic throttle actuator even if the vehicle doesn't have ASR.)

The common element in virtually every successful 'cure' of these problems comes down to pulling the codes and using that info to track down the source of the problem. In my case, a faulty brake light switch (i.e., on the brake pedal) was enough to cause the system to go into 'limp home' mode. As 994s2c eloquently points out, MB integrated a lot of functions into this system. It's actually pretty elegant when you get to know it (although frustrating when you have the sort of intermittant problems you've been having).

As far as I know, there isn't a single 'magic bullet' type of fix that'll cure your problem. You need more diagnostic info to proceed in a rational manner. I've never actually pulled the codes myself but others on the site have (as you'll see when you do the search). I paid an independent tech for an hour of diagnostic time, which led to the $15 brake switch diagnosis -- money well spent!

__________________
Steve
'93 400E